Question

A moving train is brought to rest within 20 seconds by applying brakes.find u if the retardation is -1.5m/s

Answer 1

HERE
t=20s
a=-1. 5m/s^2
v=0
Applying first equation of motion:
v=u+at
0=u-30=>u=30 m/s
Note that retardation can't be - ve as if retardation would be - ve then it would be supporting the motion instead of opposing it.
Hence instead of retardation you must use the term acceleration.